1. Field of the Invention
The present invention relates to an improvement of a structure for reinforcing bead sections of radial tires provided with carcass ply composed of steel cords, particularly radial tires for heavy duty vehicles such as trucks and buses etc.
2. Description of the Prior Art
In vehicles of this type, bias tires have been primarily used, lately, there have been used radial tires each having a rigid reinforcement at its tread portion, particularly a belt reinforcement composed of metal cords such as steel cords or the like.
In general, radial tires using metal cords as a belt reinforcement have several merits such as wear resistance, puncture resistance and the like as compared with the usual bias tires. This is because a stiff belt is disposed between the tread rubber and the carcass ply, but this belt may have adverse effects on the ride of the vehicle due to the rigid reinforcing effect with the belt. Such tires include a belt layer whose cords are disposed at an angle of 10.degree. to 30.degree. with respect to the equatorial plane of the tire and circumferentially extending on the carcass ply.
Therefore, the development of such radial tires are particularly advanced for use on good roads as opposed to poorer roads. Lately, the demand for these tires is conspicuous with the remarkable improvement of roads, such as the development of the express way networks and the like.
Radial tires are often built wherein cords of carcass plies are wrapped around a bead core from the inside to the outside of tires and are arranged in radial direction of a tire or semiradial tires wherein the cords of carcass plies are inclined at an angle of 10.degree. to 30.degree. with respect to the radial direction.
Such a steel cord reinforcing layer is secured at least near the bead portion and extended upwardly from at least near the bead base portion to a given height in a radial direction, has a weak resistance to the lateral force acting on the tire. As a result, the flexible sidewall portion produces a discontinuity of rigidity near the radially outer end of the steel cord reinforcing layer which has a very high rigidity, so that when the tire is deformed under a load, peeling of the steel cord reinforcing layer and the turned up portion of the carcass ply may occur at the discontinuous area of the rigidity. Such a problem will grow and is liable to cause a separation failure of the steel cord end, which is a fatal defect in a radial tire of this type.
In order to solve this problem, there has hitherto been proposed a method of arranging an additional organic fiber cord layer outside or inside a position corresponding to the radially outer end of the steel cord reinforcing layer and the like with respect to the axial direction of tire. Two typical embodiments of the said prior art will be explained in detail as follows.
One of them (Japanese Pat. No. 9674522) is characterized in that a steel cord reinforcing layer is disposed at the outside of the turned up portion of the said carcass ply and reinforcing fiber layers wherein two or more layers composed of organic fiber cords are crossed with each other, are arranged at the outside of the steel cord reinforcing layer and extend upwardly from the bead base up to the position at which the width of the tire cross section is at a maximum. The cut ends of the turned-up portion of the carcass ply are completely covered with the above mentioned reinforcing layers and a rubber layer composed of a hard rubber stock with JIS hardness greater than 80.degree. and a relatively soft rubber stock with JIS hardness of about 50.degree. to 60.degree. is disposed above a bead core 3. This layer is positioned between the carcass main portion 2 and the turned-up portion 2' of the carcass ply 2 and the reinforcing organic fiber cord layers with the rubber layer gradually decreasing the thickness towards the side wall of the tire and a substantially triangular bead section is formed.
The other such example (U.S. Pat. No. 3557860) discloses a pneumatic radial tire, in which the beads comprise at least a pair of bead core of steel wires about which the carcass plies are turned up, said carcass plies being all turned up towards the side directed towards the rim flange, and including a triangular filler made of a high rigidity rubber layer or the like above the bead core, and reinforcing elements constituted by two groups of strips of cord fabric, each group being constituted by one or more strips, the first group being placed inwardly of the bead, between the carcass plies and the triangular filler, and the second group being placed outwardly of the bead and of the carcass plies and alongside the turned-up portion of said plies, said groups of strips extending radially from a point approximately at the height of the inner periphery of the bead ring to at least one-sixth of the height of the tire section.
The tires constructed as above described have been favorably accepted by drivers as a tire which can be used under severe and heavy load condition.
Recently, some drivers have begun to use such tires under more even severe conditions due to economic conditions such that as a result, bead separation failure is sometimes induced. On the one hand, in order to substantially reduce the cost of tires, the rate of using a steel cord radial tire as a recaped tire has increased. It has become the present tendency to recap a worn tire two or three times. Under such circumstances, it is vigorously desired to eliminate bead separation failure which is vital to preserving a recapable tire carcass, and hence it is desirable to provide highly durable reinforced bead constructions; but the premature failure due to the aforementioned peeling phenomenon are not yet solved even by these preposals.